According to the data, solutions below about 20% ethanol (40 proof) will not ignite at room temperature (though 20% ethanol would ignite at slightly above room temperature, so it would not be surprising if you could light it on fire).

The concentration of ethanol in the water has to be high enough for its vapor pressure to be within flammability limits at that temperature. The vapor pressure does not increase linearly from zero to the value of pure ethanol when the alcohol percentage is increased, so it's difficult to calculate the minimum concentration without having experimental data, but the "proof" scale of alcohol percentage, where 100 proof is equal to 50% vol. of ethanol is based on the approximate minimum flammable concentration at room temperature.